"Engineers of Jihad" focuses on the prevalence of engineers in radical Islam, but the same principle applies to Kaczynski and Allen.
Gambetta and Hertog concluded that two factors converge to make engineering overrepresented in radicalization:
Cognitive style.
Engineering and STEM training cultivates a particular way of thinking that prefers precision and definitive solutions, and is uncomfortable with ambiguity and moral complexity. Chaos is very upsetting. In the case of Allen, the constant media drumbeat about Trump's chaos destroying our "Democracy" might have pushed him over the edge.
Status frustration.
Allen was Caltech-educated but apparently had to settle for a low-pay, part-time career as a college prep test tutor. That must have been very disappointing.
And if the above parallels aren't enough, Allen's possible HIV suggests there may be a Breaking Bad angle to this this story.;-)
It’s likely not just or so much the engineering training as the personality of those drawn to the profession.